Field of the Invention
The present invention relates to a composition which comprises water and a mixture of [3-(2,3-dihydroxyprop-1-oxy)propyl]silanol oligomers and ethers thereof, wherein the proportion of epoxy groups bound within Si compounds which may be present in the composition is <5 mol %, based on the molar proportion of Si in the composition. The invention further relates to a particular process for preparing a composition according to the invention and to the use thereof.
Discussion of the Background
Organofunctional silanes are molecules that are bifunctional. The alkoxy groups on the silicon can be detached in the presence of water and catalysts. This gives rise to reactive silanol groups which can enter into a chemical bond to inorganic substrates. The organofunctional portion of the molecule is bonded to the silicon by a C—Si bond and can consist of various organofunctional groups. If the organofunctional group consists of a methyl or alkyl group only, these groups cannot enter into any chemical reaction, but they can lead to a strong hydrophobic effect according to the chain length. Therefore, alkyltrialkoxysilanes are also used for hydrophobization of surfaces. Organofunctional silanes are obtainable with different organic groups. These are, for example, primary amino groups, secondary amino groups, and glycidyl ether, methacryloyl, ureido, vinyl, mercapto and isocyanato groups. One use of silanes is as adhesion promoters. For this function, they may be part of a primer solution or else be used as additive in a paint formulation.
Discussion about VOCs does not stop short of the silanes, since the hydrolysis of methoxy- or ethoxysilanes gives rise to methanol or ethanol. Furthermore, silanes cannot be used without restriction in aqueous paint formulations because they can be subject to further hydrolysis and condensation after the paint has been formulated. This gives rise to paint formulations having only very short application times. On the one hand, the performance of the paint can deteriorate after a short time or, on the other hand, there can even be gelation. One possible solution for the stability of silanes in aqueous paint formulations could be water-based silane systems. Thus, aqueous silane systems are the subject of increasing interest, since they contain only a low level of organic solvents, if any, and are therefore more environmentally friendly. Furthermore, these systems can be used without explosion protection. Stable aqueous silane systems are not preparable simply by mixing silanes with water, since many silanes are insoluble in the aqueous phase and undergo hydrolysis and condensation on contact with water.
EP0675128 teaches a process for preparing stable aqueous silane compositions. In the process, a water-soluble silane and a water-insoluble silane are hydrolysed and condensed in a defined molar ratio. The alcohol of hydrolysis is removed from the mixture after the reaction. The aqueous silane compositions claimed contain amino groups.
EP0849332 describes a process for producing a coating. Example 3 shows a preparation scheme for a mixed 3-glycidyloxypropyltrimethoxysilane system. The mixed system contains, as a further silane, a succinic anhydride silane. The system is applied to the substrate as coating and thermally cured.
WO1991/019565 discloses an adsorption material for selective removal of lipoprotein and cholesterol from aqueous liquids, especially from blood, plasma or serum, wherein porous glass bodies serve as solid support material for the adsorption material and organic functional groups are covalently bonded as ligands (ligands=Lig) to the surface thereof. Said ligands have alkyl radicals containing at least one ethyl moiety with a terminal alpha-/beta-diol group and are bonded to the surface of the solid support material via a Lig-Si—O bond; moreover, they do not have any free silanol groups either. For production of such adsorption materials, it is possible to use 3-glycidyloxypropyltrimethoxysilane inter alia.
WO2002/050191 discloses the production of low-solvent sol-gel systems comprising the process steps of:
a) hydrolysis and condensation
b) addition of water until phase separation
c) removal of the condensate phase.
In the hydrolysis and condensation, it is also possible to use 3-glycidyloxypropyltrimethoxy- or -ethoxysilanes.
EP1599551 claims a process for coating a metallic surface with an aqueous composition containing at least one hydrolysable or partly hydrolysed silane, a metal chelate, an organic film former and a long-chain alcohol. The silane used here may also be 3-glycidyloxypropyltrimethoxy- or -ethoxysilane.
EP1599551 teaches a process for coating a metallic surface with an aqueous composition containing at least one hydrolysable or/and at least partly hydrolysed silane and at least one metal chelate.
EP1599616 discloses a process for coating a metallic surface with an aqueous composition, wherein the composition comprises at least one hydrolysable or/and at least partly hydrolysed fluorine-free silane and at least one hydrolysable or/and at least partly hydrolysed fluorine-containing silane.
WO2005/090502 describes a binder consisting of an aqueous film-forming polymeric siloxane. The binder may contain alkyl, alkenyl, methacryloyl, epoxy, mercaptan or hydroxyalkyl groups. The binder is formulated with additives and fillers and used for the corrosion protection of metals.
WO2006/010388 teaches the preparation of a binder from a glycidyloxypropyltrialkoxysilane, a silica sol, a catalyst and a propyl zirconate or butyl titanate or titanium acetylacetonate as crosslinker. The binder can be used in formulations or else for coating of a wide variety of different substrates.
WO2005/108520 discloses an adhesive/sealant composition consisting of a component A and a component B. Component A contains at least one catalyst for silane crosslinking and low molecular weight organofunctional silanes, for example 3-glycidyloxypropyltrimethoxysilane.
WO2006/079516 relates to an aqueous binder composition consisting of an epoxysilane, a formylaminopropyltrialkoxysilane and a tetraalkoxysilane. The silanes are subjected to acid hydrolysis and formulated with pigments and additives. The aqueous coating system is intended for use as a shop primer.
WO2009/021776 describes the preparation of an oligomeric epoxysilane and optionally a further silane by hydrolysis and condensation in the presence of boric acid.
EP0675128 teaches the preparation of stable water-based silane systems. In Example 3, a 3-glycidyloxypropyltrimethoxysilane is reacted with diethylene glycol and butyl titanate. The methanol formed is removed before the converted silane is hydrolysed by addition of water.
CN103509188 describes the preparation of coloured polysiloxane microparticles. For this purpose, silanes are hydrolysed in water or in a water/solvent mixture between 20 and 80° C. and a pH of 1.0 to 6.5 in the presence of an organic acid. Subsequently, the pH is adjusted to 7.1 to 13.5 with a base. After 0.1 to 24 hours, the microparticles can be removed and dried.
US2007/0179268 claims a process for preparing oligomeric epoxysilanes. Fewer than 1.5 equivalents of water are used for the hydrolysis.
WO2006/044340 teaches the production and use of an abrasion-resistant coating composition. The composition contains at least one epoxysilane.
JP54063176 claims a primer for transparent plastics, consisting of a hydrolysed epoxysilane and an aminoalkylsilane.
JP2000239644 describes the composition of a primer which also contains epoxysilanes as well as an organic solvent and water.
EP0832911 discloses organopolysiloxane-containing water-based compositions that are essentially free of organic solvents. Studies on a composition based on 3-glycidyloxypropyltrimethoxysilane prepared according to Example 1 show that the composition, as well as a high proportion of intact epoxy groups, also contains opened epoxy groups. The result is surprising.
Aqueous silane systems are becoming ever more important, since they are environmentally friendly and easy to handle. In the use of these systems, it is not normally necessary to observe any special safety precautions with regard to explosion protection. Such aqueous systems can be used as a reactive binder, co-binder, as primer and also as sealant. Silanes are very reactive and react in the presence of air humidity or water. First of all, hydrolysis takes place. This forms silanol groups which can react with inorganic or else organic substrates. Furthermore, the silanols can also react with themselves to form Si—O—Si bonds. Under appropriate conditions, significant crosslinking in the aqueous solution and hence gelation of the system can take place. If organofunctional silanes are to be used for aqueous silane systems, the stability of the organofunctional group should also be taken into account. Example 1 in EP0832911 leads to an aqueous silane system with opened and intact epoxy groups. After the synthesis, about 20-30% of the epoxy groups are still intact.